In the fields of medical diagnosis and industrial nondestructive inspection, an X-ray examination device such as an X-ray computed tomographic photographing device (X-ray CT device) is used. The X-ray CT device has a structure in which an X-ray tube (X-ray source) to emit fan beam X-rays in a fan shape and an X-ray detector having many X-ray detecting elements arranged in parallel are arranged opposite each other with a tomographic surface of a specimen as a center. In the X-ray CT device, the fan beam X-rays are emitted from the X-ray tube to the X-ray detector, and, for Example, an angle is changed by one degree at a time with respect to the tomographic surface every time the emission is performed, and thereby collection of X-ray absorption data is performed. The X-ray absorption data are analyzed in a computer to calculate an X-ray absorptance at each position of the tomographic surface, and thereby an image according to the X-ray absorptances is constituted.
In the X-ray detector of the X-ray CT device, a solid scintillator to emit visible light by stimulation with X-rays is used. The solid scintillator is a single crystal scintillator or a polycrystalline ceramic scintillator. The development of an X-ray detector combined with such a solid scintillator and a photodiode is promoted. By using the X-ray detector using the solid scintillator, it is easy to increase the number of channels by downsizing the detecting element, and therefore high resolution of the X-ray CT device can be achieved.
As the solid scintillator to be used for the X-ray detector, there are known single crystal bodies such as cadmium tungstate (CdWO4), sodium iodide (NaI), and cesium iodide (CsI), and polycrystalline ceramics such as europium-activated barium fluoride chloride (BaFCl:Eu), terbium-activated lanthanum oxybromide (LaOBr:Tb), thallium-activated cesium iodide (CsI:Tl), calcium tungstate (CaWO4), cadmium tungstate (CdWO4), europium-activated gadolinium oxide (Gd2O3:Eu), and praseodymium-activated gadolinium oxysulfide (Gd2O2S:Pr).
Rare earth oxysulfide ceramics such as (Gd1−xRrx)2O2S (0.0001≦x≦0.01) and (Gd1−x−yPrxCey)2O2S (0.0001≦x≦0.01, 0≦y≦0.005) have properties of a large X-ray absorption coefficient and a short afterglow time of light emission. Apart from this, rare earth oxide ceramics having a garnet structure are also known as a solid scintillator. The rare earth oxide ceramics having a garnet structure have a property of excellent light output. However, in the X-ray CT device, an exposure dose of a subject is desirably further decreased. Therefore, the solid scintillator is required to achieve higher sensitivity and to decrease an afterglow time in order to shorten a scanning time.
On the other hand, in the security fields of a baggage inspection device and the like at an airport as antiterrorism measures attracting attention recently, a solid scintillator made of a single crystal body of cadmium tungstate is frequently used. The single crystal body of cadmium tungstate is inferior in properties to rare earth oxysulfide ceramics and rare earth oxide ceramics having a garnet structure, but is superior in cost to them because it is low in cost. However, the single crystal body of cadmium tungstate has a risk of worsening the environment because cadmium is a noxious substance. With regard to the rare earth oxide ceramics having a garnet structure, improvement in light output is achieved by improvement of an oxide composition or the like, but further improvement in properties is required.